1. Field of the Invention
The present invention relates to an optical receiver preamplifier, and more particularly to improved high dynamic range preamplifier.
2. Description of the Prior Art
A fiber receiver make use of the optical receiver preamplifier, the major function is that convert a current signal which is detected by a photo-diode into an amplified voltage signal. Then the signal flows into the post limiting amplifier. The transimpedance amplifier is the popular structure in the optical receiver preamplifier (shown in FIG. 1). As illustrated in FIG. 1, a transimpedance amplifier includes an input transister 10, an output transister 20, a load resistor 30, a feedback resistor 40 and a current signal source 50. The input current (I.sub.IN) is applied to the gate of transistor 10 by the input node 12 and the output voltage (V.sub.OUT) is derived from the source output node 22 of transistor 20, and therefore when the input current (I.sub.IN) of transimpedance amplifier exceeds a certain value, the current signal source 50 shuts off and the circuit becomes inoperative. In order to solve the problem of limiting the dynamic range by the transimpedance feedback resistor. Traditionally, the prior art solution to the aforementioned problem has two methods (shown in FIG. 2): Automatic Gain Control and DC Restore Function. In other word, making use of feedback circuit to control the gain of the amplifier. While signal is strong, reducing the gain of the main amplifier through the feedback circuit, and while signal is weak, increasing the gain of the amplifier through the feedback circuit. Then sent the signal to the detector for profit by detecting (not shown in the drawings). It can increase high dynamic range by applying the control theorem. Nevertheless, the two methods must work on the output node by a detector to detect the input signal. Furthermore, through the low pass filter, comparator, and feedback to control the transimpedance feedback resistor. Due to the feedback control circuit both of the two methods, not only increase the complication and area of the circuit, but also induce more noise by the resister and transistor. It also causes the instability of the feedback control circuit. So the two methods must require much cost in practical design and application.